Cyanide-free electroplating bath for the deposition of gold and gold alloys

ABSTRACT

Cyanide-free electroplating baths for deposition of gold and gold alloy coatings, using sulphurous gold complexes that are stable for a relatively long time, can be used with current density over 1 A/dm 2  and are practically odor-free, are obtained when the sulphurous compounds used are mercaptosulfonic acids, dye sulfide sulfonic acids or salts thereof.

This application is a 35 U.S.C. 371 National Stage filing ofPCT/EP97/03903 published as WO 98/03700 on Jan. 29, 1998.

DESCRIPTION

This invention relates to a cyanide-free, electroplating bath for thedeposition of gold and gold alloy coatings, containing 0.5 to 30 g/l ofgold in the form of a complex of a sulphurous compound, 0 to 50 g/l ofan alloy metal in the form of water-soluble compounds of silver, copper,indium, cadmium, zinc, tin, bismuth, arsenic and/or antimony, 1 to 200g/l of the free sulphurous compound, 0 to 200 g/l of conductive andbuffer salts in the form of alkali metal borates, phosphates, citrates,tartrates and/or gluconates and optionally wetting agents andbrighteners.

Today, electrodeposition of gold is primarily performed usingelectrolytes based on gold cyanide complexes which, at least underalkaline conditions, also contain relatively large quantities of toxicalkali metal cyanides. Under acidic and neutral conditions, the cyanideliberated on electrolysis escapes at least in part as highly toxichydrocyanic acid. Apart from severe toxicity, baths containing cyanidealso occasion problems when detoxifying the cyanide, which, in practice,is predominantly performed with alkali metal hypochlorite. This mayresult in the formation of so-called adsorbable halogen compounds (AOX)which cause waste water treatment problems. Efforts have accordinglylong been made to produce gold electroplating baths without using thetoxic complexing agent cyanide. However, with the exception of bathsbased on gold sulphite complexes, it has not hitherto proved possible toproduce an industrially viable bath.

However, such gold/sulphite complexes have the disadvantage of lowstability and, even with a large excess of free sulphite ions, formelemental gold once the solution has stood for a relatively extendedperiod, so rendering the solution unusable.

Electroplating baths which contain the gold in the form of athiosulphate complex (DE-PS 24 45 538) are also not substantially morestable. Like other known gold complexes with sulphurous compounds, theydecompose partially if they are kept for a relatively extended period.In published application EP 0 611 840, the gold thiosulphate complexesare thus stabilised by the addition of sulphinates. The current densityusable in these baths is limited and decomposition generally occurs atcurrent densities of above 1 A/dm². Moreover, these baths usually causean odour nuisance.

The object of the present invention was accordingly to provide acyanide-free electroplating bath for the deposition of gold and goldalloy coatings, containing 0.5 to 30 g/l of gold in the form of acomplex of a sulphurous compound, 0 to 50 g/l of an alloy metal in theform of a water-soluble compound of silver, copper, indium, cadmium,zinc, tin, bismuth, arsenic and/or antimony, 1 to 200 g/l of the freesulphurous compound, 0 to 200 g/l of conductive and buffer salts in theform of alkali metal borates, phosphates, citrates, tartrates and/orgluconates and optionally wetting agents and brighteners, which was alsoto be stable over a relatively extended period, to be operated atcurrent densities of above 1 A/dm² and, to the greatest extent possible,to be neutral in odour.

SUMMARY OF THE INVENTION

This object is achieved according to the invention by the bath'scontaining the gold complex of a mercaptosulphonic acid, adisulphidesulphonic acid or mixtures of these compounds as thesulphurous compound.

The salts, preferably the alkali metal salts, of these compounds arealso suitable.

The baths preferably contain 1 to 200 g/l of the free sulphurouscompound or the alkali metal salts thereof in excess of thestoichiometric composition of the corresponding gold complex.

It is furthermore advantageous for the baths to contain 0.01 to 10 g/lof wetting agents in the form of surfactants and 0.1 to 1000 mg/l ofbrighteners in the form of selenium and/or tellurium compounds.

The bath is advantageously operated at a pH value of 7 to 12.

The sulphurous compounds which are suitable for the baths according tothe invention exhibit good solubility in water and elevated stability,combined with a low vapour pressure, such that there is no perceptibleunpleasant odour.

DETAILED DESCRIPTION OF THE INVENTION

The sulphurous compounds to be used according to the invention may becharacterised by the general formula I

    X--S--CHR--(CR'R").sub.n --SO.sub.3 H                      (I)

in which

X means H or the residue --S--CHR--(CR'R")_(n) --SO₃ H R' and R"

R means H, alkyl or aryl containing up to 12 C atoms, SO₃ H, OH, SH, NH₂

n means the numbers from 0 to 6.

Typical compounds of the formula I are:

2-mercaptoethanesulphonic acid,

3-mercaptopropanesulphonic acid,

2,3-dimercaptopropanesulphonic acid and homologues together with

bis-(2-sulphoethyl)disulphide

bis-(3-sulphopropyl)disulphide and homologues.

The compounds are preferably used in the form of the alkali metal saltsthereof.

The corresponding gold complexes are obtained by simply reacting solublegold compounds, such as for example tetrachloroauric acid, sodium auratesolutions or the like, with the stoichiometric quantity or an excess ofthese sulphurous compounds in an aqueous solution. Care must be taken toprovide the stoichiometric quantity of sulphurous compounds required forreduction to gold(I). If the electroplating bath is to contain nochloride ions, the gold should first be precipitated with ammoniasolution as fulminating gold, thoroughly washed and dissolved in anaqueous solution of the sulphurous compound.

The solution of the gold complex may be used directly for preparation ofthe electroplating bath. The bath preferably contains an excess ofsulphurous compounds of 1-200 g/l.

Codeposition of further metals as well as gold from this system ispossible in order to influence properties of the deposit. Codepositionof silver, copper, indium, cadmium, tin, zinc, bismuth and thesemi-metals arsenic and antimony is of interest. They are used either inthe form of the corresponding sulphur compounds, as is preferably thecase with silver and copper, or in the form of other complexes withhydroxyl ions, with nitrilotriacetic acid or ethylenediaminetetraaceticacid (EDTA), as complexes with hydroxycarboxylic acids, such as gluconicacid, citric acid and tartaric acid, as complexes with dicarboxylicacids, such as oxalic acid, with amines, such as ethylenediamine, withphosphonic acids, such as 1-hydroxyethanediphosphonic acid,aminotrimethylenephosphonic acid orethylenediaminetetramethylenephosphonic acid.

Bismuth is thus preferably used as a citrate or EDTA complex, tinpreferably as oxalatostannate(IV) or tin(II) gluconate complex andindium as gluconate or EDTA complex. Arsenic and antimony are largelyused to increase hardness and for brightening. Arsenic is preferablyused in the form of alkali metal antimonyltartrate. The concentration ofthe alloy metals may vary within broad limits between 10 mg/l and 50g/l. The concentration of the free complexing agent in the bath may bebetween 0.1 and 200 g/l.

Bright alloy deposits may be obtained by adding further brighteners,such as compounds of selenium and tellurium, for example as an alkalimetal selenocyanate, selenite or tellurite, in concentrations of 0.1mg/l to 1 g/l.

The addition of conductive and buffer salts, such as borates,tetraborates, phosphates, citrates, tartrates or gluconates of thealkali metals, in concentrations of 1-200 g/l increases the conductivityand throwing power of the bath.

The addition of 0.01-10 g/l of wetting agent not only reduces surfacetension, but may also have a positive effect on brightening. Wettingagents used are, for example, ionic and nonionic surfactants of theethylene oxide adduct type, such as alkyl (fatty acid) or nonylphenolpolyglycol ethers with alcohol, sulphate, sulphonate or phosphate endgroups together with perfluorinated compounds, such as perfluoroalkanecarboxylates or sulphonates, together with cationic surfactants, forexample tetraalkyammonium(sic)perfluoroalkane sulphonates.

According to the invention, the baths thus contain:

0.5-30 g/l of gold complexed with sulphurous compounds from the groupcomprising mercaptosulphonic acids or the alkali metal salts thereof.

1-200 g/l of free sulphurous compounds or the alkali metal salts.

0-200 g/l of conductive and buffer substances from the group comprisingalkali metal borates, phosphates, citrates, tartrates, gluconates.

0-50 g/l of alloy metals from the group comprising silver, copper,cadmium, indium, tin, zinc, bismuth, arsenic and antimony in the form ofthe stated complexes and compounds.

0-1000 mg/l of brighteners from the group comprising selenium andtellurium in the form of selenite, selenocyanate or telluride of thealkali metals.

0-10 g/l of wetting agents, for example ionic and nonionic surfactantsof the ethylene oxide adduct type such as alkyl(fatty acid) ornonylphenol polyglycol ethers having alcohol, sulphate, sulphonate orphosphate end groups together with perfluorinated compounds such asperfluoroalkane carboxylates or sulphonates together with cationicsurfactants, for example tetraalkyammonium(sic)perfluoroalkanesulphonates.

The following Examples are intended to illustrate the invention ingreater detail:

1. 5 g of gold as gold 2-mercaptoethanesulphonate complex, 20 g of2-mercaptoethanesulphonic acid and 50 g of dipotassium phosphate aredissolved to yield a liter of solution. The pH value is adjusted to pH10 with sodium hydroxide solution. On subsequent electrolysis at 50° C.and 1.5 A/dm², a smooth, uniformly adhering gold coating of a thicknessof 5 μm is obtained on a copper cathode. The bath is virtually odourlessand exhibits no signs of decomposition even after relatively extendedelectrolysis.

2. If 2 g of a copper 2-mercaptopropanesulphonic acid complex are addedto the bath from Example 1, reddish gold/copper deposits are obtained onelectrolysis.

3. If a further 0.3 g of potassium oxalatostannate(IV) and 400 μg ofpotassium selenocyanate are added as brighteners to the bath fromExample 2 and electrolysis performed at 50° C. and 2 A/dm², bright,rose-coloured coatings of a gold/copper/tin alloy are obtained.

4. 4 g of gold and 2 g of silver as bis-(3-sulphopropyl)disulphidecomplexes, 30 g of (3-sulphopropyl)disulphide, 50 g of sodium gluconateand 20 mg of potassium tellurite are dissolved to yield a liter ofsolution. The pH value is adjusted to pH 12 with sodium hydroxidesolution. On electrolysis at 55° C. and 1.5 A/dm², a bright,greenish-yellow, ductile deposit of a gold/silver alloy is obtained on acopper cathode.

We claim:
 1. A cyanide-free electroplating bath for the deposition ofgold and gold alloy coatings, comprising0.5 to 30 g/l of gold, by weightof the gold, in the form of a gold complex, said complex being a complexof gold and sulfur-containing compound; 0 to 50 g/l of an alloy metal,by weight of the alloy metal, said alloy metal being in the form of awater-soluble compound of an element selected from the group consistingof silver, copper, indium, cadmium, zinc, tin, bismuth, arsenic,antimony and mixtures thereof; 1 to 200 g/l of a free sulfur-containingcompound; and 0 to 200 g/l of a conductive and buffer salt, wherein thegold complex of at least one member selected from the group consistingof a mercaptosulfonic acid or salt thereof, a disulfidesulfonic acid ora salt thereof, and mixtures of these compounds, and the freesulfur-containing compound is at least one member selected from thegroup consisting of a mercaptosulfonic acid or a salt thereof, adisulfidesulfonic acid or a salt thereof, and mixtures of thesecompounds.
 2. The cyanide-free electroplating bath according to claim 1,wherein said conductive and buffer salt is selected from the groupconsisting of alkali metal borates, phosphates, citrates, tartrates,gluconates and mixtures thereof.
 3. The cyanide-free electroplating bathaccording to claim 1, wherein the sulfur-containing compound of thecorresponding gold complex contains a compound of the formula I

    X--S--CHR--(CR'R").sub.n --SO.sub.3 H

in which x represents H or the residue --S--CHR--(CR'R")_(n) --SO₃ H; R,R'and R" represent H, alkyl or aryl containing up to 12 carbon atoms,SO₃ H, OH, SH, or NH₂ ; and n represents a number from 0 to
 6. 4. Thecyanide-free electroplating bath according to claim 3, wherein thesulfur-containing compound of the corresponding gold complex contains atleast one member selected from the group consisting of2-mercaptoethanesulfonic acid, 3-mercaptopropanesulfonic acid,2,3-dimercaptopropanesulphonic acid, bis-(2-sulfopropyl)disulfide,bis-(3-sulfopropyl)disulfide and the alkali metal salts thereof.
 5. Thecyanide-free electroplating bath according to claim 3, which contains0.01 to 10 g/l of wetting agent in the form of a surfactant and 0.1 to1000 mg/l of brightener in the form of selenium and/or telluriumcompounds.
 6. The cyanide-free electroplating bath according to claim 1,wherein the sulfur-containing compound of the corresponding gold complexcontains at least one member selected from the group consisting of2-mercaptoethanesulfonic acid, 3-mercaptopropanesulfonic acid,2,3-dimercaptopropanesulfonic acid, bis-(2-sulfopropyl)disulfide,bis-(3-sulfopropyl)disulfide, and the alkali metals salts thereof. 7.The cyanide-free electroplating bath according to claim 6, whichcontains 0.01 to 10 g/l of wetting agent in the form of a surfactant and0.1 to 1000 mg/l of brightener in the form of selenium and/or telluriumcompounds.
 8. The cyanide-free electroplating bath according to claim 1,which contains 0.01 to 10 g/l of wetting agent in the form of asurfactant and 0.1 to 1000 mg/l of brightener in the form of seleniumand/or tellurium compounds.
 9. A process for the electrodeposition ofgold and gold alloys from a cyanide-free electroplating bath, comprisingimmersing a cathode in a bath according to claim 1 and subjecting saidbath and cathode to electrolysis to deposit a gold or gold alloy of saidcathode.
 10. The process according to claim 9, wherein the bath isoperated at a pH value of from 7 to
 12. 11. The process according toclaim 10, wherein the bath is operated at a current density above 1A/dm² at a temperature of 50 to 55° C.
 12. The process according toclaim 9, further comprising codepositing gold with an element selectedfrom the group consisting of silver, copper, indium, cadmium, tin, zinc,bismuth, arsenic and antimony.